Not applicable.
1. Field of the Invention
The present invention generally relates to catalysts and processes for the catalytic partial oxidation of hydrocarbons (e.g., natural gas), for the preparation of a mixture of carbon monoxide and hydrogen using a supported metal catalyst. More particularly, the invention relates to syngas production processes employing catalysts having a diffusion barrier layer between a metal support and a catalytically active species.
2. Description of Related Art
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted to hydrocarbons.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widespread process, or by dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, proceeding according to Equation 1.
CH4+H2OCO+3H2xe2x80x83xe2x80x83(1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue.
The catalytic partial oxidation of hydrocarbons, e.g., natural gas or methane to syngas is also a process known in the art. While currently limited as an industrial process, partial oxidation has recently attracted much attention due to significant inherent advantages, such as the fact that significant heat is released during the process, in contrast to steam reforming processes.
In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 2.
CH4+1/2O2CO+2H2xe2x80x83xe2x80x83(2)
This ratio is more useful than the H2:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol and to fuels. The partial oxidation is also exothermic, while the steam reforming reaction is strongly endothermic. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes. The syngas in turn may be converted to hydrocarbon products, for example, fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes by processes such as the Fischer-Tropsch Synthesis.
The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors, but one of the most important of these factors is the choice of catalyst composition. Difficulties have arisen in the prior art in making such a choice economical. Typically, catalyst compositions have included precious metals and/or rare earths. The large volumes of expensive catalysts needed by prior art catalytic partial oxidation processes have placed these processes generally outside the limits of economic justification.
For successful operation at commercial scale, the catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, and the selectivity of the process to the desired products of carbon monoxide and hydrogen must be high. Such high gas hourly space velocities are difficult to achieve at reasonable gas pressure drops, particularly with fixed beds of catalyst particles. Accordingly, substantial effort has been devoted in the art to the development of catalyst support structures and the design of the catalytic reaction zone.
Fixed reaction zone processes, wherein the reaction zone comprises a fixed bed of solid catalyst particles, have been known for some time and are described in the patent literature. For example, U.S. Pat. No. 5,149,464 describes such a process and catalyst. A number of other process regimes have been proposed in the art for the production of syngas via partial oxidation reactions. For example, the process described in U.S. Pat. No. 4,877,550 employs a syngas generation process using a fluidized reaction zone. Such a process however, requires downstream separation equipment to recover entrained supported-nickel catalyst particles.
To overcome the relatively high pressure drop associated with gas flow through a fixed bed of catalyst particles, which can prevent operation at the high gas space velocities required, various structures for supporting the active catalyst in the reaction zone have been proposed. U.S. Pat. No. 5,510,056 discloses a monolithic support such as a ceramic foam or fixed catalyst bed having a specified tortuosity and number of interstitial pores that is said to allow operation at high gas space velocity. The preferred catalysts for use in the process comprise ruthenium, rhodium, palladium, osmium, iridium, and platinum. Data are presented for a ceramic foam supported rhodium catalyst at a rhodium loading of from 0.5-5.0 wt %.
U.S. Pat No. 5,648,582 also discloses a process for the catalytic partial oxidation of a feed gas mixture consisting essentially of methane. The methane-containing feed gas mixture and an oxygen-containing gas are passed over an alumina foam supported metal catalyst at space velocities of 120,000 hr.xe2x88x921 to 12,000,000 hr.xe2x88x921 The catalytic metals exemplified are rhodium and platinum, at a loading of about 10 wt %.
U.S. Pat. No. 5,744,419 (Choudhary et al.) describes certain Ni and Co catalysts on an inert support, the surface of which is precoated with an oxide of Be, Mg or Ca. These catalysts are employed for converting methane to syngas.
U.S. Pat. No. 5,338,488 (Choudhary et al) describes certain composite catalysts having the general formula TmAOn. T is a transition metal (including Ni, Co, Pd, Ru, Rh and Ir), A is an alkaline earth metal (including Mg, Ca, Ba and Sr), O is oxygen, m is the T/A mole ratio from 0.01-100 and n is the number of oxygen atoms needed to form a catalyst composite wherein each element has a complete set of valence electrons. These catalysts are said to have activity for catalyzing the production of synthesis gas by oxidative conversion of methane.
Hofstad et al. (Catalysis Today 40:157-170 (1998)) describe certain alumina supported rhodium catalysts with activity for catalyzing the partial oxidation of methane to synthesis gas.
As mentioned above, the partial oxidation of methane is a very exothermic reaction, and temperatures at typical reaction conditions in excess of 1,000xc2x0 C. may be required for successful operation. It is known that ceramic monolith catalyst supports are susceptible to thermal shock; that is, either rapid changes in temperature with time or substantial thermal gradients across the catalyst structure. Catalysts and catalyst supports for use in such a process must therefore be very robust, and avoid structural and chemical breakdown under the relatively extreme conditions prevailing in the reaction zone.
U.S. Pat. No. 5,639,401 discloses a porous monolithic foam catalyst support of relatively high tortuosity and porosity, preferably comprising at least 90 wt % zirconia for thermal shock resistance. The catalytically active components exemplified are rhodium and iridium, at a catalyst loading of 5 wt %.
Complete oxidation of hydrocarbons, such as occurs in automobile catalytic converters, also require catalysts which function at high space velocities and also are stable at elevated temperatures of greater than about 700xc2x0 C. U.S. Pat. No. 5,511,972 discloses a catalyst structure that is effective under the severe conditions encountered in automobile catalytic converters. The catalyst structure comprises a ferrous alloy as the catalyst support. The ferrous alloy contains aluminum, which forms micro-crystals or whiskers of alpha-alumina on the alloy surface when heated in air. A washcoat of gamma-alumina is added to the alpha-alumina surface followed by the deposition of palladium.
As disclosed by Czech, et al., in Surface and Coatings Technology, 108-109 (1998) p. 36-42, stationary gas turbine engines for electric power generation operate at gas inlet temperatures that are as high as those in the catalytic partial oxidation reaction zone. The turbine blades are subjected to very high thermal and mechanical loads and are additionally attacked by oxidation. To deal with the mechanical loads, the base material of the turbine blades is metallic in composition. To deal with the thermal and chemical stresses, the turbine blades have a coating with a composition represented by MCrAlY, where M comprises Ni and/or Co, as a protective overlay coating against oxidation. Additional coatings may be added as thermal barriers. The overlay coatings are typically applied by either Low Pressure Plasma Spray or Vacuum Plasma Spray. The base material is protected in operation by an alumina scale, which forms from the overlay coating.
There remains a need for a process for the catalytic partial oxidation of hydrocarbons, particularly methane, that provides high levels of conversion of methane and high selectivities for CO and H2 products. An economical catalyst, with good thermal and mechanical stability and that permits economical operation at low pressure drop is needed for use in such a process.
The present invention provides a process and catalyst for the catalytic partial oxidation of a hydrocarbon feedstock, and a method for preparing the catalyst. The process and catalyst overcome many of the deficiencies of previous syngas processes and catalysts.
The new process comprises the catalytic partial oxidation of a hydrocarbon feedstock by contacting a feed stream containing a hydrocarbon feedstock and an O2-containing gas with a catalyst in a reaction zone maintained at conversion-promoting conditions effective to produce an effluent stream comprising carbon monoxide and hydrogen.
In accordance with the invention, a preferred catalyst used in the process comprises a partial oxidation catalyst containing a catalytically active component selected from the group consisting of rhodium, platinum, ruthenium, iridium, rhenium, and combinations thereof, supported on a catalyst support comprising iron, nickel or cobalt and an oxide surface layer.
Another catalyst in accordance with the invention comprises a partial oxidation catalyst comprising a catalytically active component selected from the group consisting of rhodium, platinum, ruthenium, iridium, rhenium, and combinations thereof, supported on a ferritic catalyst support comprising an aluminum-containing oxide-dispersion-strengthened alloy and an oxide surface layer.
Another catalyst in accordance with the invention comprises a catalyst selected from the group consisting of rhodium, platinum, ruthenium, iridium, rhenium and combinations thereof, on a support prepared by heating in oxygen at about 1000xc2x0 C. a ferritic stainless steel alloy consisting essentially of 15 to 25 weight % chromium, 3 to 6 weight % aluminum, 0.1 to 1 weight % yttrium oxide and the balance iron, for a time sufficient to grow a thin, compact oxide layer on the alloy surface.
The invention also provides a method for the preparation of a supported partial oxidation catalyst comprising treating a catalyst support comprising iron or nickel or cobalt at an elevated temperature in an oxygen-containing atmosphere to form an oxide surface layer, and supporting a catalytically active metal for partial oxidation on the treated support.
Also provided in accordance with the invention is a method of converting a reactant gas mixture comprising C1-C5 hydrocarbons and O2 into a product gas mixture comprising H2 and CO. In certain embodiments, the H2 and CO are in a molar ratio of about 1.5:1 to about 2.3:1, preferably about 2:1. The method includes contacting the reactant gas mixture at partial oxidation promoting conditions of temperature, pressure and feed flow rate with a catalyst comprising a catalytically active component selected from the group consisting of rhodium, platinum, ruthenium, iridium, rhenium, and combinations thereof, supported on a catalyst support. The catalyst support may be (1) an oxide-dispersion-strengthened (ODS) alloy comprising aluminum, chromium, and yttrium oxide, at least one metal selected from the group consisting of iron, nickel, and cobalt, and, optionally, titanium, or the support may be (2) a non-ODS alloy comprising chromium, aluminum, titanium, an element selected from the group consisting of yttrium, lanthanum and scandium, and at least one metal selected from the group consisting of iron, nickel and cobalt. The catalyst has a metal oxide layer disposed between the catalytically active component and the support.
Still other embodiments, features and advantages of the present invention will appear from the following description.
The term xe2x80x9ccatalytic partial oxidationxe2x80x9d when used in the context of the present syngas production methods, in addition to its usual meaning, can also refer to a net catalytic partial oxidation process, in which hydrocarbons (comprising mainly methane) and oxygen are supplied as reactants and the resulting product stream is predominantly the partial oxidation products CO and H2, rather than the complete oxidation products CO2 and H2O. For example, the preferred catalysts serve in the short contact time process of the invention, which is described in more detail below, to yield a product gas mixture containing H2 and CO in a molar ratio of approximately 2:1. Although the primary reaction mechanism of the process is partial oxidation, other oxidation reactions may also occur in the reactor to a lesser or minor extent. As shown in Equation (2), the partial oxidation of methane yields H2 and CO in a molar ratio of 2:1.
The process of the present invention is used to prepare a mixture of carbon monoxide and hydrogen from any gaseous hydrocarbon having a low boiling point by catalytic partial oxidation of the hydrocarbon. The gaseous hydrocarbon is preferably methane, natural gas, associated gas or other sources of light hydrocarbons having 1 to 5 carbon atoms. Natural gas is mostly methane, but it can also contain up to about 25 mole % ethane, propane, butane and higher hydrocarbons. Natural gas from naturally occurring reserves can also contain carbon dioxide, nitrogen, hydrogen sulfide, and other minor components.
The hydrocarbon feedstock is in the gaseous phase when contacting the catalyst. Preferably, the feed comprises at least 50% by volume methane, more preferably at least 75% by volume methane, and most preferably at least 80% by volume methane. The hydrocarbon feedstock is contacted with the catalyst as a mixture with an oxygen-containing gas, preferably pure oxygen. The methane-containing feed and the oxygen-containing gas are mixed in such amounts to give a carbon (i.e., carbon in methane) to oxygen (i.e., oxygen) ratio from about 1.25:1 to about 3.3:1, more preferably from about 1.3:1 to about 2.3:1, and most preferably from about 1.5:1 to about 2.2:1. The process of the present invention may be operated at atmospheric or super-atmospheric pressures, with the latter being preferred. The process may be operated at pressures of from about 101 kPa to about 3000 kPa, and preferably from about 850 kPa to about 3000 kPa. Preferably the flow rate of the reactant gas mixture is maintained at about 100,000 hrxe2x88x921 or more. The process may be operated at temperatures of from about 600xc2x0 C. to about 1300xc2x0 C., and preferably from about 800 xc2x0 C. to about 1200xc2x0 C. The hydrocarbon feedstock and the oxygen-containing gas are preferably pre-heated from about 50xc2x0 C. to about 700xc2x0 C., more preferably about 400xc2x0 C.
The hydrocarbon feedstock and the oxygen-containing gas can be passed over the catalyst at a variety of space velocities. Preferred space velocities for the process, stated as normal liters of gas per kilogram of catalyst per hour, are from about 60,000 to about 20,000,000 NL/kg/h, preferably from about 150,000 to about 10,000,000 NL/kg/h. Ceramic foam monoliths are typically placed before and after the catalyst as radiation shields. The inlet radiation shield also aids in uniform distribution of the feed gases.
A preferred catalyst used in the process of the present invention comprises rhodium, platinum, ruthenium, iridium, rhenium, and combinations thereof, on a metallic support. The most preferred catalyst comprises rhodium on a metallic support. Suitable metallic supports for use in the present invention are in the form of gauzes, honeycombs, spiral rolls of corrugated sheet, columnar or other configurations having longitudinal channels or passageways permitting high space velocities with a minimal pressure drop. Such configurations are known in the art and described in, for example, Structured Catalysts and Reactors, A. Cybulski and J. A. Moulijn (Eds.), Marcel Dekker, Inc., 1998, p. 599-615 (Ch. 21, X. Xu and J. A. Moulijn, xe2x80x9cTransformation of a Structured Carrier into Structured Catalystxe2x80x9d) incorporated herein by reference in pertinent part.
Suitable supports can be prepared from high temperature oxidation-resistant, aluminum-containing oxide-dispersion-strengthened (xe2x80x9cODSxe2x80x9d) alloys. These alloys contain a dispersion of an oxide, such as Y2O3. Oxide particles serve to strengthen the alloy and promote the formation of a compact, tenacious, oxide layer on the alloy surface when properly treated. One preferred ODS alloy for use as a catalyst support with the present invention consists of, by weight, 15 to 25% chromium (Cr), 3 to 6% aluminum (Al), 0.1 to 1.0% titanium (Ti), 0.1 to 1.0% Y2O3 and the balance iron (Fe). These alloys are designated Fe-base ODS alloys and are readily commercially available. Other preferred ODS alloys are the Ni-base ODS alloys and Co-base alloys.
Fe-base or Ni-base or Co-base alloys that do not contain an oxide dispersion but contain Cr and Al can also be satisfactorily used as catalyst supports in the present invention. One preferred alloy of non-ODS composition consists of, by weight, 15 to 25% chromium (Cr), 3 to 6% aluminum (Al), 0.1 to 1.0% titanium (Ti), 0.3 to 1.0% yttrium, lanthanum or scandium (Y, La or Sc), and the balance iron (Fe) or nickel (Ni) or cobalt (Co).
The catalyst support is preferably pretreated by heating in air or oxygen at 900 to 1200xc2x0 C., preferably 1100xc2x0 C., for from 10 to 100 hours, preferably 50 hours, to form a thin, tightly adhering oxide surface layer that protects the underlying support alloy from further oxidation during high temperature use. The surface layer also functions as a diffusion barrier to the supported catalyst metal (e.g. Rh, Pt, Ir, Ru, Re and combinations thereof), thus preventing alloying of the catalyst metal with the alloy of the catalyst support. The protective surface layer is preferably composed predominantly of alpha-alumina, but may also contain a small amount of yttrium oxide.
After pretreatment, the catalyst supports are coated with a catalyst metal such as Rh, Pt, Ru, Ir, Re, and combinations thereof, preferably Rh. The coating may be achieved by any of a variety of methods known in the art, such as physical vapor deposition, chemical vapor deposition, electrolysis metal deposition, electroplating, melt impregnation, and chemical salt impregnation, followed by reduction.
Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent. The following embodiments are to be construed as illustrative, and not as limiting the disclosure in any way whatsoever.